This invention relates generally to the filtering of fluids under pressure and more particularly to such a system which provides for bypass of the filter responsive to its becoming clogged and, in addition thereto, means for reverse flow of the fluid also bypassing the filter.
Various systems are known which require the flow of fluid under pressure from a source thereof to a load and also from the load to the source. Typical of such systems are the hydrostat transmission system composed of a hydraulic pump and a hydraulic motor coupled together in a closed fluid flow loop.
It is typical in such systems to provide a filter interposed to clean the fluid flowing from the source to the load. The filter removes foreign materials from the fluid which may be suspended therein from various sources. As the filter tends to become clogged from the removal of the suspended material, the flow through the filter diminishes while the differential fluid pressure across the filter increases. In hydrostat transmission systems which typically operate at quite high pressures, if the clogging becomes substantial, the filter may rupture dumping its load of contaminants into the flow of fluid to the load. It is thus desirable if the filter element is not changed prior to becoming substantially clogged to provide a bypass for flow from the source to the load responsive to an increase beyond a predetermined threshold of the differential fluid pressure across the filter element.
At the same time, when flow is from the load to the source it is desirable for the fluid to not pass through the filter. If such flow does pass through the filter it would unload the material removed by the filter and carry those foreign particles back into the source or pump and possibly deposit some of them in the motor or load with resultant destructive action on their moving parts. Therefore it is customary to provide a bi-directional valve in the system to provide bypass flow around the filter should it become clogged as well as to provide reverse flow through a path which also bypasses the filter.
As used in this application the term "forward flow" is applied to the direction of flow of the fluid through a given fluid path of the system from the source or pump to the load. "Reverse flow" is flow of fluid from the load or the motor to the source or the pump while the term "bypass flow" is referred to as that flow from the source to the load but not through the filter.
There are various bi-directional valves currently in existence for use in such systems. Typical of such systems are those disclosed in U.S. Pat. Nos. 3,497,929, 3,487,932, 3,625,248, 3,807,442, 3,908,693, 3,996,137 and 4,439,984.
While the systems including the valves shown in the prior art patents above referred to operate well under most conditions, they are complex in construction, expensive to manufacture, provide a slow response to change in direction of flow of the fluid from forward to reverse and, in some instances, the bypass valves are separate from the reverse flow valves. Because of the stringent demands upon a bi-directional valve in such a system to preclude malfunctioning of the system, the design of a bi-directional valve for inclusion in such a system which is capable of operating in a confined space and which can operate quickly to provide for the full flow immediately upon change of flow direction as required by a hydrostatic transmission system has proved to be a complex and perplexing problem, particularly when coupled with the requirements for low cost and ease of installation and maintenance.